A solvent system for making cellulosic food casings is known in the art. Briefly, a natural cellulose such as a wood pulp or cotton linters is mixed with an aqueous tertiary amine oxide cellulose solvent such as N-methyl-morpholine-N-oxide (NMMO) to prepare a premix. The premix is heated with stirring to evaporate a portion of the water and dissolve the cellulose. The result is a thermoplastic cellulose solution, called "dope", which can be extruded or molded to a desired shape. The extruded or formed article composed of the dope then is contacted with a cellulose non solvent such as water or a non solvent mixture of water and NMMO. The contact with the non solvent extracts the solvent from the dope causing the precipitation or regeneration of the cellulose from the solution.
The solvent system, as set out above, is distinguished in that the natural cellulose is dissolved directly as opposed to chemically treating the cellulose to form a soluble cellulose derivative as in the well known viscose process. Accordingly, for purposes of the present invention, the term "non derivatized cellulose" means a cellulose which has not been subject to covalent bonding with a solvent or reagent but which has been dissolved by association with a solvent or reagent through Van der Waals forces or hydrogen bonding.
The dissolution process as described above is more particularly set out in U.S. Pat. Nos. 2,179,181; 3,447,939; 4,145,532; 4,426,228 and 5,094,690. Methods and apparatus for forming a food casing from the thermoplastic cellulose solution are more particularly described in U.S. Pat. Nos. 5,277,857; 5,451,364; 5,759,478 and 5,766,540.
In brief, forming a tubular food casing is accomplished by extruding the dope as a thin walled tube about a mandrel and into a bath of non solvent liquid or "outer bath" which contacts the outer surface of the extruded tube. As noted above, the extraction of solvent and regeneration of the cellulose in the bath forms a hydrated, or "gel", tube of cellulose. After regeneration, the gel tube is washed to remove traces of the solvent. Then it is dipped into a glycerine solution and dried under inflation to about 7 to 10 percent moisture based on the weight of cellulose to form a tubular cellulose film for use as a sausage casing. After drying, the inflated tube (hereafter referred to as casing) is collapsed to its flat width and reeled with up to 11,000 feet (3353 meters) or more of casing on each reel. The casing on the reel is referred to as "semi-finished casing". The semi-finished casing then is subjected to further treatment in order to prepare it for use by the sausage maker.
During the course of the downward extrusion, a non solvent liquid is introduced into the interior of the extruded tube. This introduced liquid forms an internal bath for initiating the extraction of the solvent at the inner surface of the extruded tube. The introduced liquid also cools the interior surface of the extruded tube and provides lubrication between the extruded tube and the mandrel. At some point during its decent into the outer bath, the extruded tube must be collapsed to a flat width so it forms a web for further transport through the process. Collapsing the tube to a flat width holds back the inner bath and prevents most of the inner bath from being carried forward through subsequent steps in the process.
A conventional flattening ladder used, for example, to flatten a blown film, is not effective to flatten the extruded tube because it is not able to hold back the liquid of the inner bath. A pair of nip rolls to collapse the extruded tube also can not be used. This is because the extruded tube at this point in the manufacturing process is still relatively fragile and the pressure applied to create the nip unacceptably damages the extruded tube by creasing or folding it. Simply passing the extruded tube around a turn roll or draw roll that directs the extruded tube upward and out of the outer bath will collapse the tube but is unacceptable because the tube tends to slip with respect to the roll and this alters aspects of the casing geometry.
Another aspect to consider is that the means to collapse the tube to a flat width should not interfere with the speed of the extrusion at start-up. In this regard, the extrusion speed may be 60 meters per minute or more. Thus, the means used to flatten the tubular extrusion must be applied in a manner which does not slow the extrusion rate.
Means found to be effective both to collapse the tube to a flat width and hold back the liquid of the internal bath comprise a plurality of rods around which the tube is laced. Lacing the tube around the rods directs the tube through a sinuous path or "S-wrap" which collapses the tube to a flat width. Moreover, it was found that if the rods are of sufficient diameter, the flattening can be accomplished without creasing the tube. However, while the S-wrap is effective to flatten the tubing and to hold back the internal bath liquid without creasing the casing, lacing the extruded tube around the rods to create the S-wrap is difficult to accomplish at extrusion speeds. Such lacing is particularly difficult because it must be done at some depth below the level of the outer bath.
Accordingly, one object of the present invention is to provide a method and apparatus for collapsing an extruded tube of non derivatized cellulose to a flat width without applying a nip to the tube.
Another object of the present invention is to provide a method and apparatus for collapsing an extruded tube of non derivatized cellulose to a flat width for holding back an internal bath liquid with out creasing the extruded tube.
A further object of the present invention is to provide a method and apparatus for collapsing an extruded tube of non derivatized cellulose to a flat with by lacing a leading edge of the tube around a lacing means to create an S-wrap.
Still another object of the present invention is to provide a method and apparatus for flattening an extruded tube of a non derivatized cellulose to a flat width by creating an S-wrap lacing of the tube during the extrusion of the tube without decreasing the speed of the extrusion or distorting the fragile extruded tube.